A stereolithography process for producing a three-dimensional object is disclosed in JP-A-56-144478 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), which comprises applying a requisite amount of controlled light energy to a liquid photohardenable resin to harden the irradiated resin in a thin layer, supplying the liquid photohardenable resin on the hardened layer, followed by irradiation under control to harden the resin in a thin layer to be integrally superposed on the previously formed layer, and repeating these steps until a desired solid three-dimensional object is built up. Ever since a basic process in which the stereolithography technique can be put to practical use has been proposed in JP-A-60-247515, a number of proposals concerning stereolithography have been made, as disclosed, e.g., in JP-A-62-35966, JP-A-1-204915, JP-A-1-213304, JP-A-2-28261, JP-A-2-80422, JP-A-2-80423, JP-A-2-113925, JP-A-2-145616, JP-A-2-208305, JP-A-2-153722, JP-A-3-21432, JP-A-3-41126, JP-A-5-5004, JP-A-5-279436, JP-A-6-228413, and JP-A-6-228271.
A typical and commonly used process for producing a three-dimensional object by stereolithography comprises selectively applying a computer-controlled ultraviolet (UV) laser beam to the liquid surface of a liquid photohardenable resin composition in a container according to a designed pattern to harden the resin to a prescribed depth (thickness), supplying the liquid resin composition on the hardened layer to the thickness corresponding to one layer, applying a UV laser beam thereto to successively form a hardened layer on the preceding layer, and repeating these steps until a three-dimensional object having the entire three-dimensional object has been built up. This process has recently been attracting attention because three-dimensional objects having complicated shapes can be obtained easily in a relatively short time.
With the recent broadening of application of stereolithography from concept models to test models and trial products, there has been an increasing demand to provide three-dimensional objects having still higher dimensional accuracy and dimensional stability. The objects have also been demanded to have excellent mechanical properties.
Photohardenable resin compositions generally used in stereolithography comprise at least one photo-polymerizable compound, such as a photo-polymerizable modified urethane (meth)acrylate compound, an oligoester acrylate compound, an epoxyacrylate compound, an epoxy compound, a polyimide compound, an aminoalkyd compound, and a vinyl ether compound, as a main component, and a photosensitive polymerization initiator. For the purpose of improving dimensional accuracy, dimensional stability, and mechanical characteristics, various proposals have been made in the above-enumerated publications with regard to the kinds of the photo-polymerizable compounds and initiators, etc., but the aims have not yet been accomplished sufficiently.
Further, for the purpose of preventing shrinkage of a three-dimensional object obtained by stereolithography, there has been proposed the addition into a photohardenable resin composition of an additive that is soluble in the composition while being separated from the composition on hardening to form a different phase (as described in JP-A-3-20315) or the addition of a polymeric coagulating material which coagulates when heated (as described in JP-A-3-104626). However, it is difficult to select the additive, which can form different phases on hardening or to select the polymeric coagulating material, which coagulates when heated, and further it is difficult to dissolve the additive or the polymeric coagulating material stably in the photohardenable composition. Moreover, because the additive or the polymeric coagulating material forms a different phase on hardening in the photohardened resin composition, there are disadvantages that a transparent three-dimensional object cannot be obtained, and that the continuous phase cannot be firmly formed to induce the phase separation, which reduces the mechanical strength of a hardened product. Therefore, these processes have not brought about advantageous effects.
On the other hand, it is necessary to increase the hardening reaction rate in order to form a three-dimensional object by stereolithography in a short time with high productivity. For this purpose, irradiation with high energy for photopolymerization is required. However, if the applied light energy is very high, the penetration depth in the direction of irradiation (Z-axis) is larger than necessary and tends to be nonuniform. It follows that the hardening of the resin in the Z-axis is nonuniform, the thin layer formed of the hardened resin has an uneven thickness, and the three-dimensional object formed by laminating of infinite thin layers of uneven thickness will have markedly deteriorated dimensional accuracy and dimensional stability, particularly in the Z-axial direction. That is, an increase in reaction rate in hardening and an improvement in dimensional accuracy or dimensional stability of the resulting three-dimensional object are conflicting with each other, difficult to be satisfied simultaneously.
Under these circumstances, there have been made various studies and proposals concerning apparatus, methods of control on light irradiation, and the formulations of a photohardenable resin composition to be used in stereolithography to control and to make the Z-axial penetration depth uniform, while increasing the reaction rate of the resin and reducing the time of reaction by applying high light energy. Among the proposals concerning the material, there is the addition of a polarizing substance, which is capable of polarizing (scattering) light applied in the Z-axis to other directions including the X-axis and the Y-axis, into a photohardenable resin composition, as described in JP-A-3-15520, JP-A-3-41126, JP-A-3-1147732, and JP-A-3-114733. According to this process, light having proceeded in the Z-axis is struck against a light polarizing substance (light scattering substance) present in the photohardenable resin composition and thus polarized (scattered). As a result, the unnecessary penetration of the light into the Z-axial direction is avoided, and the resin is hardened more uniformly in the Z-axial direction than in the conventional techniques.
In this technique, however, light which has been struck against the light polarizing substance (light scattering substance) and polarized (scattered) reaches over the prescribed boundaries at the XY surface, making it difficult to control the hardening of the resin in other directions including the X- and Y-axial directions perpendicular to the Z-axis. It is likely that the scattered light induces unnecessary photo-hardening, which results in obscure boundaries on the XY surface between the area to be irradiated and those which should not be irradiated, failing to provide a three-dimensional object with a clear outline. Light irradiation could be controlled under considering the quantity of light scattered over the XY surface, but such control would be extremely complicated. In addition, the process necessitates previous addition of particulate light polarizing substance (light scattering substance) to a liquid photohardenable resin composition or addition of a substance capable of separating from the resin matrix phase upon photohardenable. As a result, a transparent object is hardly obtainable. If a colored object is intended, the color tone tends to be whitened and unclear.